Allied Telesis AT-S39 - Manual (Page 101)

AT-S39 User’s Guide
101
Forwarding Delay and Topology Changes
If there is a change in the network topology due to a failure, removal, or
addition of any active components, the active topology also changes.
This may trigger a change in the state of some blocked ports. However, a
change in a port state is not activated immediately.
It might take time for the root bridge to notify all bridges that a topology
change has occurred, especially if it is a large network. If a topology
change is made before all bridges have been notified, a temporary data
loop could occur, and that could adversely impact network
performance.
To forestall the formation of temporarily data loops during topology
changes, a port designated to change from blocking to forwarding
passes through two additional states, listening and learning, before it
begins to forward frames. The amount of time a port spends in these
states is set by the forwarding delay value. This value states the amount
of time that a port spends in the listening and learning states prior to
changing to the forwarding state.
The forwarding delay value is adjustable on the AT-8000 Series switch
through the management software. The appropriate value for this
parameter will depend on a number of variables, with the size of your
network being a primary factor. For large networks, you should specify a
value large enough to allow the root bridge sufficient time to propagate
a topology change throughout the entire network. For small networks,
you should not specify a value so large that a topology change is
unnecessarily delayed, which could result in the delay or loss of some
data packets.
Note
The forwarding delay parameter applies only to STP.
Hello Time and Bridge Packet Data Units (BPDU)
The bridges that are part of a spanning tree domain communicate with
each other using a bridge broadcast frame that contains a special
section devoted to carrying STP or RSTP information. This portion of the
frame is referred to as the Bridge Packet Data Unit (BPDU). When a
bridge is brought on-line, it will issue a BPDU in order to determine
whether a root bridge has already been selected on the network. and if
not, whether it has the lowest bridge priority number of all the bridges
and should therefore become the root bridge.